Metabolism and Heat
Exercise involves more strenuous use of the muscles than what is involved in everyday activities. Therefore, these muscles need more energy than they usually do. When the body releases energy from nutrients, heat is released in the process. This excess heat needs to be removed from the body so the body can remain at a safe temperature. Sweating is one form of heat loss, as the sweated moisture uses heat energy to evaporate off into the ambient air, thereby removing some heat from the surface of the skin. However, increasing the blood flow under the skin is the reason skin becomes redder or pinker during exercise.
Initial Skin Changes
When a muscle begins exercising, it needs more blood than normal to carry extra nutrients and oxygen and remove extra metabolic wastes. This extra blood comes from areas of the body that are not involved in the exercise at that time. Blood is diverted from the internal organs to the muscles, for example, and at the beginning of the exercise session, the skin even shunts some of its blood to the muscles.
According to a 2011 review in the "Experimental Physiology" journal, the core temperature of the person exercising continues to rise without blood flow increasing to the skin up to a certain threshold temperature. This threshold temperature varies with the individual and training can reduce the threshold at which blood flow increases to the skin. The mechanism by which more blood flows in the skin is vasodilation, which is a widening and relaxation of the blood vessels that allows more blood to pass through. The heat from the muscles is carried by the blood, away from the muscles and to the skin to dissipate.
The blood contains haemoglobin, which is red in color and carries oxygen. As blood flow increases to the skin, the skin becomes pinker or redder in color due to more blood than normal passing through. The reason the skin appears uniformly pinker is because the blood is carried through tiny blood vessels called capillaries which are not visible under the skin as individual vessels.